More specifically, the invention relates to the field of alkali-metal (cesium) cells which may be very small, for example having a volume of a few cm3, for which a typical application is a chip-scale atomic clock known by the acronym CSAC. Such cells and atomic clocks are sometimes called “micro-cells” or “micro-clocks”. Throughout the following text, the term “micro” is to be understood as having the context and meaning indicated.
Such micro-clocks are intended, for example, for use in telecommunications, navigation, and defense. But the alkali-metal vapour cells concerned can also be used in sensors such as micro-magnetometers and micro-gyroscopes.
The micro-clocks considered here are based on the principle of atomic resonance by coherent population trapping, known by the acronym CPT.
Research has yielded several CSAC prototypes since 2004, specifically in the context of the NIST program (acronym for National Institute of Standards and Technology). More recently, Symmetricom has commercially released the CSAC known as the SA 45s, which has a volume of 16 cm3, weighs 35 g, and has a power consumption of only 120 mW
As part of the MAC-TFC consortium, the FEMTO-ST Institute (acronym for Franche-Comté Electronique Mécanique Thermique et Optique—Sciences et Technologie) began work in 2008 on designing and building a very compact cesium vapour cell (a few mm3) with MEMS (acronym for MicroElectroMechanical Systems) machining of silicon and anodic bonding. The cell is defined on the sides by a first glass cover and a second glass cover, spaced apart from one another and arranged parallel to one another. The cell forms a sealed vacuum cavity which is filled with cesium by making use of a locally heated alkali dispenser, which overcomes the problem of the conflict between the anodic bonding and the chemistry of cesium (see “New approach of manufacturing and dispensing of micromachined cesium vapor cell” of L. Nieradko, C. Gorecki, A. Douahi, V. Giordano, J. C. Beugnot, J. Dziuban, and M. Moraja published in the JOURNAL OF MICRO-NANOLITHOGRAPHY MEMS AND MOEMS of August 2008). This cell has an architecture referred to as “transmissive”, the laser, in this case a vertical cavity surface emitting laser known by the acronym VCSEL, and the photodetector (photodiode) being located one on either side of the cell itself, the laser beam traversing it from side to side, entering through the first glass cover and exiting through the second glass cover.
The prior art is also illustrated by documents EP 0550240, EP 2154586, EP 2362282, U.S. Pat. No. 6,265,945, U.S. Pat. No. 6,320,472, US 2002/0163394, US 2009/0251224, JP 2007 178273, and JP 2007 178274, which describe different arrangements which all have an architecture where the laser beam traverses the cell, as above.
Document US 2005/0046851 aims to provide CSACs that are more compact, less complex, and less expensive than existing ones. It describes an architecture in which the VCSEL and the photodetector are integrated and form an assembly located at the first end of the cell, while a flat reflective surface is provided at the second end of the cell, the two ends being planar, spaced apart from each other, and arranged parallel to one another. With this architecture, the VCSEL produces a diverging laser beam which passes through the cell a first time in order to reach the flat reflective surface, and then after reflection passes through the cell a second time in the opposite direction in order to reach the photodetector. This architecture has the disadvantage of requiring a diverging laser beam and of leading to two passes through the cell in two opposite directions.
There is therefore the need for gas cells (alkali-metal vapour cells) specifically designed for atomic clocks such as CSACs, having an architecture that improves their performance in terms of compactness, frequency stability, power consumption, and integration of clock components, and achieving this with an assembly process that is easier, more precise, and suitable for industrial scale production.